What fuel does your body prefer? (ii)

The millionaire, Malibu-dwelling, ex-professional marathon runner Mark Sisson, ‘the godfather of Paleo‘, doesn’t have much in common with me, your broke, east London-dwelling, ex-triathlete blogger, ‘the stepson of zero-carb‘. But we both used to do a lot of endurance sport, we both ate tons of carbs to facilitate it, and the overcarbing had negative effects on both of us.

Sisson, with a marathon PB of 2hrs 18 (a tad faster than my PB), running 100-mile training weeks, says, ‘I was…a picture of health: 6% body fat, resting heart rate of 38 beats per minute…’

The truth was different: ‘…my training regime and state-of-the-art high-energy diet...resulted in a succession of serious overuse injuries, illnesses and burnout. While I looked the picture of health, I was really one of overstress and inflammation. …I suffered from recurring bouts of fatigue, osteoarthritis in my feet, severe tendonitis in my hip joints, stress-related gastrointestinal maladies and six or more upper respiratory tract infections each year.’

Me too, Mark, me too. Apart from the 6% body fat. Anyway, we’re both ok now, and both living la vida low-carb. Or zero-carb. Neither Mark nor I do endurance, and nor do we do carbs any more.

In my last post on this subject, I remarked upon the fact that taking in carbs for fuel is only effective if that’s what you typically do, and in that way it can be argued, as conventional scientific wisdom does, that carbs are our ‘preferred’ fuel source.  But does the human body really ‘prefer’ carbs when there is an honest choice?

Here’s a clue: most mammals use fat for fuel. Cows and gorillas, herbivores both, for example, have digestive systems that convert cellulose to triglycerides. They eat plants but burn fat. We do not have the huge factories in our guts that the herbivores do; our systems are more like those of dogs, for example, evolved to deal efficiently with meat and animal fat. As carnivores, we can eat fat rather than manufacturing it, and we do very well metabolising it for fuel.

Our fuel supplies from fat are very efficient and long-lasting, but we won’t be able to access them if we are constantly stuffing carbs in our faces. (This is why many studies by sports scientists on fuel use seem to show better results on carbs – none of these studies allowed sufficient time for the subjects to become fat-adapted).

More signs of efficient fat use: fat cells not only have a great capacity to store fat, they can do so using minimal water – glycogen storage inefficiently requires double the amount of water, ie every gram of glycogen requires two grams of water. So we never have more than a few hours’ worth of glycogen to play with, whereas if push comes to shove we can move for days on our fat stores.

The primary obstacle to efficient fat-burning is the presence of insulin. Insulin inhibits the breakdown of fat, because one of its main roles is storage. If insulin levels are always high, due to the constant intake of carbs, then fat usage will simply not take place. And in a particularly unfair trick of nature, small rises in insulin cause huge drops in  fat release.

So how can we get off the carb-insulin-carb cycle and start accessing all those fat stores to fuel our efforts? Two to three weeks of eating a carb-restricted diet will do the trick. Phinney and Volek have published studies in which athletes were given four to six weeks adaptation that show ‘a progressively increased capacity to mobilise and oxidise fat [at threshold]’.

Jeff Volek, co-author of ‘The Art and Science of Low -Carbohydrate Living’ and a groundbreaking researcher into this area, has stated that the uniformity of response to carb restriction in athletes points very strongly to this being an evolutionary trait. In other words, we are hard-wired to respond in a certain way to carb restriction.  And I mentioned dogs above, with regard to similarities in metabolism: Phinney and Volek point out that sled dogs, especially those racing 100-mile days in events such as the Iditarod, are able to replenish muscle glycogen on their zero-carb diets, relying, as humans can, on circulating lipids and ketones for amazing feats of strength and endurance.

Our bodies prefer fat as fuel. Just as we require no carbohydrates to live a healthy life, neither do we need carbs to fuel our efforts. As long as we allow ourselves to adapt to our true fat-burning nature, we’re good to go.

Eat steak, run like a wolf!

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